The phenomenon of superconductivity--that is, zero electrical resistance--which is exhibited by many metals at low temperatures, is of great scientific and commercial value, since it permits various sorts of high-powered devices to operate with minimal losses of electrical power. The phenomenon is defeated by the exceeding of any one of three parameters: a critical temperature (T.sub.c), a critical magnetic field (H.sub.c), and a critical current (J.sub.c). Since the most interesting and useful applications of superconductivity involve high currents or fields, it has been the aim of the art of superconductor manufacture to produce conductors with the highest possible critical quantities. One material with useful superconducting properties is the alloy NbTi; alloys of Nb-40-60 at. % Ti are typically used.
It has been well known in the art for some time that one way to increase the operating parameters of a given superconductor is to "stabilize" it by providing a normally conductive alternate current path, so that if the superconductor should "go normal"--i.e., return to the normally conducting state--the current will have a shunt path. In this way higher currents can be passed through the superconductor without fear of momentary local variations in the current or the magnetic field destroying the superconductor. Furthermore, the shunt provides the superconductor with a time in which it can regain its superconductive properties.
It is also well known in the art that the tendency of superconductors to go normal is ordinarily a local phenomenon. Hence, it is important to locate the shunt in close physical proximity to the superconductor, in order that the whole of the current need not be diverted.
Bearing these points in mind, the superconductor designer, when selecting a stabilizing material, has these factors to consider: the electrical resistance of the material must be as low as possible, so that if it need carry current, it will generate as little heat as possible, so that the superconductor returns from the normal state quickly and so that less stabilizer is required; its thermal conductivity should be high, so that any heat generated may be rapidly dispersed away, and also to save time in cooling the assembly to cryogenic temperatures before use of the superconductor; and it is preferably readily penetrable by magnetic flux, so as to reduce flux heating.
The material which best combines the above-listed characteristics is high-purity aluminum (99.999% pure). This material, though, while it offers the additional advantages of light weight, ready availability, and moderate cost, is not without its drawbacks. In particular, it has heretofore been impossible to mechanically co-work pure Al with the elements, compounds or alloys which have the most useful superconducting properties--e.g., NbTi or Nb.sub.3 Sn--since these alloys are very much harder and have a much greater tensile strength than aluminum. Hence, in the prior art, in the mechanical steps which are usually performed--e.g., extrusion, drawing, rolling, and swaging--the aluminum did not stay in its original configuration with respect to the superconducting materials.
It is well known in the superconductor art that the critical current density J.sub.c of a given material is dependent on the form of the specimen under test, and that wires of small diameter (i.e., on the order of micrometers) perform much better than larger ones. Since wires of such microscopic dimensions are difficult to fabricate, the practice has been to encase a number of rods in a billet of suitable nonsuperconducting material and draw the whole to what would ordinarily be considered fine wire size.
Thus, although it has been possible in the prior art to stabilize a monofilamentary superconductor with aluminum (see, for example, U.S. Pat. No. 3,514,850, "Electrical Conductors", issued to Barber et al) this has not been sufficient to satisfy the needs of the art; a multifilamentary conductor is needed.
It is therefore an object of the invention to provide an aluminum-stabilized multifilamentary superconductor.
It is a further object of the invention to provide a method whereby such a superconductor can be made readily and economically.
Other aspects and objects of the invention will appear to those skilled in the art.